1. Field of the Invention
This invention relates to a fuel injection system for a diesel engine, and more particularly to a fuel injection system for a diesel engine having a pintle fuel injection nozzle which has a valve needle lifted by the fuel pressure and in which the effective area of the spray hole or the spray area is changed according to the lift of the valve needle. In this specification, the term "pintle fuel injection nozzle" should be broadly interpreted to include throttle type nozzles in which the so-called "throttle range" in which the valve needle throttles the spray hole extends over a relatively large part of the lift of the valve needle.
2. Description of the Prior Art
As one type of pintle fuel injection nozzle, there has been known a central plunger type pintle fuel injection nozzle as disclosed in Japanese Unexamined Patent Publication No. 57(1982)-151058. In the central plunger type pintle fuel injection nozzle disclosed in this Patent Publication, a plunger member is provided behind a valve needle to be slidable coaxially with the valve needle, and the valve opening pressure at a predetermined lift of the valve needle (pre-lift) is increased stepwise by imparting a predetermined pressure to the plunger member, thereby suppressing lift of the valve needle and controlling the injecting pressure and the like to improve combustion in the combustion chamber.
In the central plunger type pintle fuel injection nozzle, the plunger member is spaced away from the valve needle by a distance corresponding to the amount of the prelift, and the valve needle is lifted by a relatively small valve opening pressure which may be of a small value such that it can just overcome the force of a spring until the valve needle is lifted to abut against the plunger member, while after the valve needle abuts against the plunger member, further lift of the valve needle is resisted by the fuel pressure imparted to the plunger member and the valve needle cannot be further lifted until the valve opening pressure is increased to overcome the sum of the fuel pressure imparted to the plunger member and the force of the spring.
By combining the central plunger type fuel injection nozzle with a pintle nozzle having a throttle range in which the front end portion of the valve needle is moved into and away from the spray hole, atomization of sprayed fuel can be promoted and the fuel injection rate (the amount of fuel injected per unit crank angle) can be controlled by extending the time the lift of the valve needle is in the throttle range, and self-control of the lift of the valve needle by the fuel pressure can be obtained.
The arrangement of self-control of the valve needle lift is advantageous in that fluctuation in the effective area of the spray hole from cylinder to cylinder due to deposition of carbon or variation in the machined dimensions of the spray holes, for instance, can be compensated for by setting the pre-lift at the boundary between the throttle range and the lift change range in which the lift of the valve needle changes with fuel pressure or in the lift change range so that the amount of lift of the valve needle (the area released) is self-controlled according to the fuel pressure. For example, in a cylinder with a spray hole which is narrower than that of the other cylinders, the injection amount is apt to become less and/or the injecting time is apt to become longer. However, in the self-control arrangement, such reduction of the injection amount and/or such extension of the injecting time can be prevented since when the spray area is narrower, the fuel pressure becomes higher whereby the amount of lift is increased to increase the injection amount and to shorten the injecting time.
However, the conventional central plunger type pintle fuel injection nozzle has a drawback in that because the amount of the pre-lift solely depends upon the fuel pressure, it can be matched to only a particular operating condition of the engine, and cannot properly use functions for atomization of fuel and/or for changing the fuel injection amount according to various operating conditions of the engine. Thus, in the conventional central plunger type pintle fuel injection nozzle, it is difficult to control the lift of the valve needle in order to obtain improved combustion in the combustion chamber and to improve the engine output power according to the operating condition over a wide operating range of the engine.